Template:Doc/doc

Using {{doc}} on a page will transclude the text of the /doc/ sub-page, if that page exists. if ti does not, it will do nothing.

On Template:spoiler, the sub-page would be Template:spoiler/doc.

{{doc}}'s purpose it to make it easier to maintain documentation, especially template- while also separating the editing history of the documentation from the editing history of the template itself.